The glass substrate used for various end uses, such as LCD substrates, must be as nearly free of defects as possible. One of the most pernicious defects is gas-filled bubbles, commonly referred to as seeds or blisters. When in the body of the glass, bubbles can refract light, thus distorting the image. For LCD substrates, when at the surface, the bubbles deform the transistor itself, and thus compromise the performance of the entire device.
The process of removing gaseous inclusions from a melt is referred to as fining or refining, and chemical constituents added to produce a fining effect are called fining agents. LCD substrates have been manufactured in the art using oxides or acids of arsenic as a fining agent. Represented as As2O5, this commonly constitutes 0.9-1.1 wt % of the glass. It is believed to achieve bubble-free glass by reducing from +5 to +3 at high temperature, after most melting is complete. This reduction releases oxygen, which fills remaining bubbles and causes them to rise out of the glass. Arsenic is an outstanding fining agent, producing virtually bubble-free glass with very little process intervention. Unfortunately, it is also a toxic element, one of the eight listed heavy metals in the Resource Conservation and Recovery Act (RCRA). The processing of the glass surface results in arsenic-bearing waste that is expensive to process. Furthermore, “cradle-to-grave” responsibility for generating arsenic-containing glass is a potential burden.
Antimony oxide (Sb2O3) is also used as a fining agent. However, it is much less effective than arsenic oxide, because it loses its oxygen at lower temperature, before melting is complete. Furthermore, antimony is closely related to arsenic in terms of its chemical behavior, and so, poses many of the same challenges for waste streams.
Tin oxide (SnO2) is not toxic and has been used as a fining agent. But its very low solubility at the forming temperature of LCD glasses (˜0.15 wt %) limit how much can be added, and therefore, compromises its efficacy.
There is a need to address the aforementioned problems and other shortcomings associated with the traditional fining agents and the traditional approaches for fining glass. These needs and other needs are satisfied by the fining agent technology of the present invention.